(announcer) the all-new subaru forester. love. it's what makes a subaru, a subaru. governor of getting it done. you know how to dance... with a deadline. and you...rent from national. because only national lets you choose any car in the aisle... and go. you can even take a full-size or above, and still pay the mid-size price. this is awesome. [ male announcer ] yes, it is, business pro. yes, it is. go national. go like a pro. yes, it is. >>> welcome back to cnn's continuing coverage of the investigation into the terrorist attacks here in boston, and, of course, the incident in texas last night with the explosion near waco. i want to give you two looks at the blast that rocked that small town in texas. these go by fairly quickly. here's the first. it's slow motion. then the shot we'll freeze. the second view occurs in realtime. just watch. enormous power behind that blast that set off seismometers. they registered 2.1. joining us now from atlanta is doug mulford, a chemistry lecturer at emory university. thank you for joining us. what is the active material in the fe